Many containers in chemical industry are coated with rubber, in order to counteract chemical, thermic or mechanical damage. These rubber coatings deteriorate with time and must be removed and replaced by new rubber coatings. The rubber can be removed with the help of chemical solvents, by burning, or by plain manual removal. These methods are time-consuming, cost intensive, and partially damaging to the environment. When the rubber is burnt off, there may also be changes in the composition of the container's materials. It has therefore been attempted to cause the rubber coating to crack by spraying it with liquid nitrogen. For this purpose, one or several ring-shaped ducts, provided with sprayers, are placed inside the containers, and through these, liquid nitrogen is sprayed onto the inside of the container. Under the influence of the cold from the evaporating nitrogen, the container wall, which is generally made of steel, and the rubber coating will shrink due to the different heat expansion coefficients of rubber. Under continued spraying with liquid nitrogen, low temperatures from -100.degree. C. to -150.degree. C. are finally reached, which cause the rubber to harden completely and to crack. The rubber coating can then no longer compensate for the ever increasing separating forces by altering its shape. The contacat binding is finally broken, without any outside mechanical force having been applied to it. When the container has returned to the surrounding temperature and the oxygen concentration inside the container permits entry, the rubber, which has been cracked from underneath, can then be removed with little effort.
This method for rubber removal works quite well for small containers. In the case of large containers, however, it has already caused material damage. These result from the uneven cooling of the large container. In large containers, cold gas will descend to the lower part of the container, due to its higher specific density. Higher up, the temperature of the evaporating gas in the container increases gradually. This causes an undesirable temperature gradation over the height of the container, which results in tensions in the construction of the container. These additional tensions are slight by themselves and can easily be removed from the container material. In combination with further tensions, however, inadmissible stresses on the container materials may result. These further tensions consist of intrinsic tensions with known intensity and direction and additional tensions from the speed of cooling. To these are now added the additional tensions from a non-uniform temperature field, during cooling and finally also in the cooled stationary condition. Under unfavorable conditions, this may result in stresses that damage or even destroy the container. It should also be considered that many container materials, e.g. ferritic steel, only have a small impact resistance at low temperatures. For these reasons, liquid nitrogen has no longer been used for the removal of rubber on large containers.